Fluid mixing and rinsing system for a flow cytometer

ABSTRACT

Disclosed is a system that can mix deionized water and concentrated sheath fluid to provide sheath fluid in a flow cytometer system having a desired concentration. Flow rates are low, which substantially match the flow rate of sheath fluid through the nozzle, so that turbulence and air bubbles are not formed in the sheath fluid. The available deionized water is then used for back flushing and removal of sample cells and deposited salts from the sheath fluid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority to U.S. provisionalapplication Ser. No. 61/663,021, filed Jun. 22, 2012, entitled “FluidMixing and Rinsing System for a Flow Cytometer,” which application isspecifically incorporated herein by reference for all that it disclosesand teaches.

This application is related to U.S. Provisional Patent Application Ser.No. 61/656,934, filed Jun. 7, 2012, by Daniel N. Fox, Susan Hunter,Nathan Michael Gaskill-Fox, Kevin P. Raley and Richard A. Miles,entitled “Automated and Accurate Drop Delay for Flow Cytometry,” U.S.Provisional Patent Application Ser. No. 61/659,528, filed Jun. 14, 2012,by Daniel N. Fox and Nathan M. Gaskill-Fox, entitled “Flow Rate Balance,Dynamically Adjustable Sheath Delivery System for Flow Cytometry,” U.S.Provisional Patent Application filed on the same date as the presentapplication, by Nathan M. Gaskill-Fox, Daniel N. Fox and Rodney C.Harris, entitled “Two Station Sample and Washing System,” U.S.Provisional Patent Application filed on the same date of the presentapplication, by Daniel N. Fox, Matthias J. G. Ottenberg and Kevin P.Raley, entitled “Condensed Geometry Nozzle for Flow Cytometry,” and U.S.Provisional Patent Application filed on the same date as the presentapplication, by Nathan M. Gaskill-Fox, Daniel N. Fox and Rodney C.Harris, entitled “Multi-Directional Sorting with Reduced Contaminationin a Flow Cytometer.” All of these applications are hereby specificallyincorporated herein by reference, for all that they disclose and teach.

BACKGROUND

Flow cytometers are useful devices for analyzing and sorting varioustypes of particles in fluid streams. These cells and particles may bebiological or physical samples that are collected for analysis and/orseparation. The sample is mixed with a sheath fluid for transporting theparticles through the flow cytometer. The particles may comprisebiological cells, calibration beads, physical sample particles, or otherparticles of interest. Sorting and analysis of these particles canprovide valuable information to both researchers and clinicians. Inaddition, sorted particles can be used for various purposes to achieve awide variety of desired results.

SUMMARY

An embodiment of the present invention may therefore comprise a systemfor mixing deionized water and sheath fluid concentrate comprising: afirst container that supplies deionized water; a second container thatsupplies concentrated sheath fluid; a pump that delivers the deionizedwater and the concentrated sheath fluid into a reservoir at a rate thatis sufficiently slow that substantially no bubbles form in thepressurized reservoir; a valve that has a first input that is coupled tothe deionized water in the first container, and supplies the deionizedwater through an output to the reservoir when the valve is in a firstposition, and a second input that is coupled to the concentrated sheathfluid in the second container, and supplies the deionized water throughthe output to the reservoir; a controller that places the valve in thefirst position until a predetermined amount of the deionized water issupplied to the pressurized container, and switches the valve to thesecond position until a predetermined amount of the concentrated sheathfluid is supplied to the reservoir, so that the predetermined amount ofthe deionized water and the predetermined amount of the concentratedsheath fluid creates a mixture having a desired concentration of sheathfluid.

An embodiment of the present invention may further comprise a method ofmixing deionized water and concentrated sheath fluid in a flow cytometercomprising: supplying deionized water from a first container; supplyingconcentrated sheath fluid from a second container; pumping the deionizedwater and the concentrated sheath fluid into a reservoir at a rate thatis sufficiently slow to substantially eliminate turbulence that causesbubbles to form in the reservoir; controlling a first valve that isconnected to the first container in a first position and the secondcontainer in a second position so that the first valve is disposed inthe first position until a predetermined amount of the deionized wateris supplied to the reservoir and the first valve is disposed in thesecond position until a predetermined amount of the concentrated sheathfluid is supplied to the reservoir to produce sheath fluid having apredetermined concentration in the reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an embodiment of a sheath fluid mixingsystem.

FIG. 2 is a flow diagram illustrating the operation of the three-wayvalve illustrated in FIG. 1.

FIG. 3 is a schematic illustration of a combined sheath fluid mixingsystem and wash system.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is schematic illustration of an embodiment of a sheath fluidmixing system 100. As illustrated in FIG. 1, deionized water 144 from adeionized water container 104, and concentrated sheath fluid 146 inconcentrated sheath fluid container 106, are supplied to a three-wayvalve 112. The deionized water container 104 may be an unpressurizeddeionized water container that is accessible to a user, so that a usercan remove the container and replace the container with a new container.Similarly, the concentrated sheath fluid container 106 is alsoaccessible to a user, so that the user may remove the concentratedsheath fluid container 106 and replace it with a new concentrated sheathfluid container. Three-way valve 112 is controlled by a valve controlsignal 124 that is generated by a level controller/rate integrator 122.Continuous pump 116 draws either deionized water 144 or concentratedsheath fluid 146 through the three-way valve 112, depending upon theposition of three-way valve 112.

Concentrated sheath fluid having a concentration of 8× has typicallybeen used to reduce shipping costs and storage costs of sheath fluid byreducing the size and weight of the shipped product by a factor of 8×.Users of flow cytometers then mix the 8× concentrated sheath fluid withdeionized water to create sheath fluid having the proper 1×concentration. However, if the deionized water and the sheath fluidconcentrate are mixed at a rate that is not slow, turbulence is createdand micro-bubbles are introduced into the mixture. These micro-bubblescan accumulate in the sheath fluid mixing system 100, illustrated inFIG. 1, and cause problems in the operation of the flow cytometer. As aresult, users are typically instructed to mix the concentrated sheathfluid and the deionized water at least a day or more before use. This isnormally an impractical solution and requires additional labor on thepart of the user. Operator usability of the system and automation of theprocesses that eliminate manual labor are advantageous.

Since supplies of concentrated sheath fluid are readily available tousers and the concentrated sheath fluid is less costly to ship andstore, it is desirable to provide a way to use the concentrated sheathfluid. Further, most laboratories that use flow cytometers have sourcesof deionized water that can be mixed with the concentrated sheath fluid.If not, pre-packaged supplies of deionized water are available. Thesystem illustrated in FIG. 1 uses the three-way valve 112 to slowly mixthe deionized water 144 with the concentrated sheath fluid 146 bycontrolling the three-way valve 112 with the valve control signal 124,so that the three-way valve supplies a predetermined amount of deionizedwater 144 and a predetermined amount of the concentrated sheath fluid146 to provide a supply of sheath fluid that has the proper (1×)concentration. Continuous pump 116 pumps the deionized water 144 andconcentrated sheath fluid 146 through the pump tube 118 and sheath inputtube 136 to the reservoir 102. Reservoir 102 may be disposed in the flowcytometer, and not be readily removed by a user. The amounts of each ofthe respective fluids is small, so that the fluids disperse in thereservoir 102 to create a uniform concentration sheath fluid 148 whichis 1×. For example, in one embodiment, a continuous pump 116 initiallysupplies fluid to the reservoir 102 at a rate of 8 mL per minute. Thisrate is designed to substantially match the outflow rate of sheath fluid148 from the reservoir 102 through the sheath delivery tube 140 to thenozzle 142. By supplying fluid to the reservoir 102 at a rate thatsubstantially matches the outflow of sheath fluid from the reservoir102, a substantially constant level of sheath fluid 147 can bemaintained in reservoir 102, as disclosed in more detail with respect toU.S. Patent Application Ser. No. 61/659,528, filed Jun. 14, 2012, byDaniel N. Fox and Nathan M. Gaskill-Fox, entitled “Flow Rate Balance,Dynamically Adjustable Sheath Delivery System for Flow Cytometry,” whichis specifically incorporated herein, by reference, for all that itdiscloses and teaches.

To create an in-flow of 8 mL per minute, three-way valve 112 provides 7mL of the deionized water 144 and 1 mL of the concentrated sheath fluid146. In this manner, 8 mL of fluid are supplied and the amount of theconcentrated sheath fluid 146 is one-eighth of the total amount offluid, and deionized water 144 is seven-eighths of the total amount offluid. The total amount of 8 mL supplied allows the concentrated sheathfluid 146 to easily disperse within the much larger volume of sheathfluid 148 in the reservoir 102. Since the injection of the deionizedwater 144 and the concentrated sheath fluid 146 matches the outflow rateof sheath fluid 148 through nozzle 142, the injection occurs over a oneminute period. The very slow rate of injection of the deionized water144 and concentrated sheath fluid 146 into the reservoir 102, a totalamount of 8 mL in a minute, results in virtually no turbulence and nobubbles being created in the sheath fluid 148. Level sensor 128 sensesthe level of the sheath fluid 148 in the reservoir 102 and generates alevel sensor signal 126 that is supplied to the level controller/rateintegrator 122. If the level of the sheath fluid 148 in the reservoir102 is low or high, the rate at which fluid is pumped by the continuouspump 116 is adjusted by the level controller/rate integrator 122 using apump speed controller signal 120. This is explained in more detail inthe above-referenced patent application, entitled “Flow Rate Balance,Dynamically Adjustable Sheath Delivery System for Flow Cytometry.”

FIG. 1 also discloses a compressor 130 that supplies compressed air toair regulator 132. The source of regulated air 134 is supplied to thereservoir 102 at a pressure of approximately 30 psi, in one embodiment.The pressurized air 150 in the reservoir 102 further regulates theamount of pressure on the fluid stream that is delivered through thesheath delivery tube 140 to nozzle 142. It is desirable to have aconstant pressure of the sheath fluid 148 flowing through the nozzle 142to accurately perform the processes of a flow cytometer. The pressure ofair 150 in the reservoir 102 makes up for differences in the pressure ofthe fluid in the sheath delivery tube 140 that results from changes inthe level of the sheath fluid 148, as set forth in the above-referencedpatent application. The adjustment to the flow of the continuous pump116 by the level controller/rate integrator 122 may affect the timing ofthe operation of the three-way valve 112. The level of the sheath fluid148 in the reservoir 102 may change for various reasons. For example,continuous pump 116 may become air locked because of a bubble in theoutput tube 114. A purge process must then occur, and the level of thesheath fluid 148 may change by an amount that results in the continuouspump 116 running at a higher rate than the rate to normally maintain thelevel of the sheath fluid 148 substantially constant in the reservoir102. In addition, the deionized water container 104 and concentratedsheath fluid container 106 may be hot swapped with new containers, whichalso causes the level of the sheath fluid 148 to decrease. Again, thisis explained in more detail with respect to the above-referencedapplication entitled “Flow Rate Balance, Dynamically Adjustable SheathDelivery System for Flow Cytometry.”

Since the pressure of the sheath fluid 148 in the nozzle 142 should beclosely maintained, any changes in the level of the sheath fluid 148 inreservoir 102 should be adjusted in a quick manner. Accordingly it isdesirable to change the flow rate of the continuous pump 116 as soon aspossible to make up for changes in the level of the sheath fluid 148 inthe reservoir 102. As such, the level controller/rate integrator 122tracks the amount of deionized water 144 supplied to the reservoir 102until a predetermined amount has been delivered. Then, the three-wayvalve 112 is changed to close the port for the supply tube 108 and openthe port for the supply tube 110 to pump the concentrated sheath fluid146 into the reservoir 102 until a predetermined amount of theconcentrated sheath fluid 146 is pumped into the reservoir 102 to createthe proper ratio and proper concentration. For example, the levelcontroller/rate integrator 122 generates a valve control signal 124 topump deionized water 144 into reservoir 102. The level controller/rateintegrator 122 generates a pump speed control signal 120 based upon thelevel sensor signal 126 generated by the level sensor 128. If the sheathfluid 148 is low, the level sensor 128 detects the low level andgenerates a level sensor signal 126, that is read by the levelcontroller/rate integrator 122, which generates the pump speed controlsignal 120 to increase the flow rate of the continuous pump 116.

The level controller/rate integrator 122, illustrated in FIG. 1, tracksthe amount of deionized water 144 pumped by the continuous pump 116 intothe reservoir 102 by integrating the pump speed of the continuous pump116 over time. In one embodiment, when it is determined that 7 mL ofdeionized water 144 has been pumped by the continuous pump 116, a valvecontrol signal 124 is generated to switch to the supply tube 110 toprovide concentrated sheath fluid 146. The level controller/rateintegrator 122 integrates the pump rate of the continuous pump 116 overtime until 1 mL of concentrated sheath fluid 146 has been pumped by thecontinuous pump 116. At that point, the level controller/rate integrator122 generates a valve control signal 124 to switch the three-way valve,so that deionized water 144 is being pumped by the continuous pump 116.This process continues. In this manner, the concentration of the sheathfluid 148 in the reservoir 102 does not change, since the proper ratioof fluids is supplied to the reservoir 102. When the continuous pump 116is pumping at higher rates to increase the level of the sheath fluid 148in the reservoir 102, the time periods for switching the three-way valve112 are reduced. Similarly, if the level of the sheath fluid 148 in thereservoir 102 is above the preset level, the continuous pump 116 has areduced flow rate and the time periods for switching the three-way valve112 are increased.

FIG. 2 is a flow chart 200 of the operation of the system illustrated inFIG. 1. At step 202, the level controller/rate integrator 122 places thethree-way valve 112 is a position so that deionized water 144 is beingpumped from the deionized water container 104. At step 204, the pumpflow rate is integrated over time to determine the amount of deionizedwater that has been pumped by the continuous pump 116. For example, inone embodiment, the pump rate is summed at 20 ns intervals. The sum ofthe pump rate over the 20 ns intervals indicates the amount of fluidthat has been pumped by the continuous pump 116. For example, at 8 mLper minute, each 20 ns interval accounts for approximately 160 pL. If 25million intervals at 20 ns are summed together, which would amount to0.5 seconds, the sum would be 4 mL. When the target amount is reached,the level controller/rate integrator 122 generates a valve controlsignal 124 to change the three-way valve 112 to pump the concentratedsheath fluid 146. Accordingly, at step 206, it is determined whether theamount of fluid, in this case the deionized water 144, is greater thanor equal to 7 mL. If not, the process continues to integrate the pumpflow at step 204. If the amount of deionized water has reached 7 mL, theprocess proceeds to step 208. At step 208, the valve control signal 124is generated to switch the three-way valve 112 to the concentratedsheath fluid 146. The process then proceeds to step 210, where the levelcontroller/rate integrator 122 integrates the pump flow for theconcentrated sheath fluid 146. The sampling can take place at the 20 nsrate, as set forth above, or at any desired rate. Further, anyintegration period can be used to provide an accurate number for theamount of concentrated sheath fluid delivered by pump 116. At step 212,it is determined whether the amount of concentrated sheath fluid isgreater than or equal to 1 mL. If not, the process returns to step 210.If the amount is greater than or equal to 1 mL, the levelcontroller/rate integrator 122 generates a valve control signal 124 toswitch the three-way valve 112 to pump deionized water at step 214. Theprocess then returns to step 204 to integrate the pump flow for thedeionized water.

Accordingly, the level controller/rate integrator 122 generates a valvecontrol signal 124 that switches the three-way valve 112 between supplytube 108 that supplies the deionized water 144 and supply tube 110 thatsupplies the concentrated sheath fluid 146. The valve control signal 124is generated when the integrated amount of flow, as detected by levelcontroller/rate integrator 122, reaches a predetermined amount for eachof the fluids. In the present instance, the amounts of 7 mL of thedeionized water 144 and 1 mL of the concentrated sheath fluid 146 areused as the predetermined amounts to achieve the proper concentration ofmixed fluid. Other amounts can be used.

FIG. 3 is a schematic illustration of a combined sheath fluid mixing andwash system 300. As illustrated in FIG. 3, reservoir 302 contains asheath fluid 374 and pressurized air 375. The deionized water container304 provides a supply of deionized water 370. Concentrated sheath fluidcontainer 306 provides a supply of concentrated sheath fluid 372.Three-way valve 312 is connected to supply tube 308, that is disposed indeionized water container 304, and supply tube 310, is disposed inconcentrated sheath fluid container 306. The level controller/rateintegrator 322 generates a valve control signal 324 that operates thethree-way valve 312 to select either supply tube 308 or supply tube 310.Continuous pump 316 operates in response to the pump control signal 320to provide a continuous flow of sheath fluid into the reservoir 302, viapump tube 318, and sheath input tube 336. Level sensor 328 senses thelevel of the sheath fluid 374 in the reservoir 302 and generates a levelsensor signal 326 that is applied to the level controller/rateintegrator 322. The pump control signal 320 controls the continuous pump316 to generally match the in-flow of sheath fluid through pump tube 318to the outflow of sheath fluid 374 through sheath uptake tube 338 andsheath delivery tube 340. If the level of the sheath fluid 374 is belowa certain predetermined level, the pump speed control signal 320increases the pumping rate of the continuous pump 316, so that thedesired level of the sheath fluid 374 can be re-established in thereservoir 302. Similarly, if the level of the sheath fluid 374 in thereservoir 302 is too high, the pump rate of the continuous pump 316 isreduced.

As also illustrated in FIG. 3, compressor 330 provides compressed air toair regulator 332. The regulated air 334, at the output of the airregulator 332, is applied to the reservoir 302 to maintain a supply ofpressurized air 375 in the reservoir 302. The air pressure of air 375 isalso regulated to maintain a substantially constant pressure of sheathfluid 374 in the sheath delivery tube 340, as described in more detailwith respect to the above-identified application entitled “Flow RateBalance, Dynamically Adjustable Sheath Delivery System for FlowCytometry.” Since the sheath fluid 374 is under pressure, the sheathfluid flows through the sheath uptake tube 338 and sheath delivery tube340 to the three-way valve 348. These devices described in FIG. 3operate in the same manner as the devices illustrated in FIG. 1, toprovide the proper concentration of the sheath fluid 374, which ismaintained by pumping the proper amounts of deionized water 370 andconcentrated sheath fluid 372 into the reservoir 302 by controlling thethree-way valve 312. Again, the pump rate of the continuous pump 316 isintegrated over short time periods to determine and control the amountof fluid delivered from each container 304, 306. In this manner, theproper concentration of sheath fluid 374 is maintained in the reservoir302.

FIG. 3 also illustrates a cleaning cycle that utilizes the deionizedwater 370 in the unpressurized external deionized water container 304.Deionized water can perform a substantially better job of cleaninginternal parts that have been in contact with sample fluid and sheathfluid. For convenience, most systems simply use sheath fluid forcleaning parts of a flow cytometer that have contacted sample fluid. Asillustrated in FIG. 3, rather than using sheath fluid, the deionizedwater 370 can be used that is available in the deionized water container304.

Deionized water is excellent fluid for dissolving solids that haveformed from sheath fluid, such as deposited salts, and also kills andremoves many cells that could cause contamination of a subsequent sampleto be sorted. Cells typically die because the cellular process attemptsto balance the salt concentration inside the cell with the environmentof the fluid in which the cell is disposed. In a deionized waterenvironment, cells absorb water to balance the salt concentration of thecells with the deionized water, which does not contain any salt. Cellsabsorb water so quickly, in an attempt to reduce salt concentration,that the cells burst and die. For these reasons, deionized water is apreferred rinsing fluid.

As shown in FIG. 3, the combined sheath fluid mixing and washing system300 provides an available source of deionized water 370 in the deionizedwater container 304. The deionized water 370 can then be used for bothdissolving salt deposits from the sheath fluid that may accumulate onparts of the system that contact sheath fluid, and kill and remove cellsthat may have deposited or collected in the flow cytometer.

During a wash phase, rinse pump 344 is activated to deliver deionizedwater 370 via rinse delivery tube 346. Deionized water is applied to afirst input port of a three-way valve 348. A second input port of thethree-way valve 348 is connected to the sheath delivery tube 340, whichsupplies sheath fluid 374 from the sheath uptake tube 338 to the nozzlesupply tube 356 during a sample cycle. During the wash cycle, thethree-way valve delivers deionized water 370 through the nozzle supplytube 356, which is connected to the nozzle cavity of the nozzle 342. Thedeionized water is inserted into the nozzle cavity of nozzle 342 underpressure, to create a stream 366 of deionized water that flows into thewaste collector 364. Any deposited salts from the sheath fluid in thenozzle 342, including the opening at the bottom of nozzle 342, aredissolved and rinsed by stream 366. The stream 366 of deionized waterflows through the waste collector 364, dissolving any salts that aredeposited in the waste collector 364, and are disposed of by waste tube362 through valve 376 and pumped by the waste pump 380 into the wastecontainer 382. The rinse delivery tube 346 also supplies deionized water370 to the valve 350. When the valve 350 is opened, deionized waterflows through the rinse tube 384. The deionized water is sprayed aroundthe outside of the sample uptake tube 354 to remove and kill any samplecells that may exist on the outside surface of the sample uptake tube354, as disclosed in more detail with respect to the above-identifiedapplication entitled “Flow Rate Balance, Dynamically Adjustable SheathDelivery System for Flow Cytometry.”

As also shown in FIG. 3, waste pump 380 then draws the deionized water370 through the waste tube 386 and valve 378 for deposit into the wastecontainer 382. In addition, the pressure of the deionized water 370 inthe nozzle 342, created by the rinse pump 344, causes the deionizedwater to flow backwards through the injector needle 352, into the sampletubing 358. Any sample that is left in the injector needle 352 isbackwashed through the sample tubing 358 and backwards through thesample uptake tube 354. Any sample fluid that remains in the sampletubing 358, or the sample uptake tube 354, is backwashed into the washstation 368. Any sample cells left from the sampling process are killedand drained through waste tube 386, valve 378, and pumped by the wastepump 380 into the waste container 382. Additionally, deionized water 370backflows through the de-bubble tube 360 and through valve 373 to thewaste pump 380, which disposes of the fluid in waste container 382.

Hence, the mixing system illustrated in FIG. 3 allows for accuratemixing at low flow rates to supply a sheath fluid of a properconcentration into the reservoir 302. The low flow rates do notsubstantially create any turbulence or bubbles in the sheath fluid 374.Accurate concentrations can be established by integrating the pump rateto determine the amount of fluid that has flowed from each of thecontainers 304, 306. By using concentrated sheath fluid, the cost ofshipping the sheath fluid is substantially reduced. Deionized water canbe easily generated at the location of the flow cytometer. The supply ofdeionized water in the system allows for convenient and easy washing toboth clear and kill sample cells and remove accumulated salts on variousportions of the flow cytometer system. All of the paths that aretypically exposed to sheath fluid and air that could dry and formdeposits can be fully washed with the deionized water. The deionizedwater scours the deposits and cleans the system to prevent deposits orcrystals from causing problems and kills and eliminates sample cellsthat may cause contamination. Because of the low flow rate, thedeionized water and the concentrated sheath fluid are mixed in a mannerthat does not create turbulence or bubbles.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andother modifications and variations may be possible in light of the aboveteachings. The embodiment was chosen and described in order to bestexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the inventionin various embodiments and various modifications as are suited to theparticular use contemplated. It is intended that the appended claims beconstrued to include other alternative embodiments of the inventionexcept insofar as limited by the prior art.

What is claimed is:
 1. A system for mixing deionized water and sheathfluid concentrate comprising: a first container to contain deionizedwater; a second container to contain concentrated sheath fluid having afirst concentration, wherein the first container and the secondcontainer are configured to simultaneously contain different fluids; apressurized reservoir; a valve that has a first input, a second input,and an output, wherein: the first input is fluidically coupled to thefirst container, the second input is fluidically coupled to the secondcontainer, the valve is configured to allow deionized water in the firstcontainer to flow through the output to the pressurized reservoir whenin a first position, and the valve is configured to allow concentratedsheath fluid in the second container to flow through the output to thepressurized reservoir when in a second position; a pump that isfluidically interposed between the valve and the pressurized reservoirand configured to flow deionized water in the first container andconcentrated sheath fluid in the second container into the pressurizedreservoir at a flowrate that is sufficiently slow that substantially nobubbles form in the pressurized reservoir; and a controller that isconfigured to maintain a concentration of a mixture of the deionizedwater and the concentrated sheath fluid in the pressurized reservoirsuch that the mixture has a concentration of sheath fluid less than thefirst concentration by repeatedly switching the valve between the firstposition, thereby supplying a first amount of deionized water by thepump to the pressurized reservoir, and the second position, therebysupplying a second amount of concentrated sheath fluid by the pump tothe pressurized reservoir.
 2. The system of claim 1 wherein thecontroller comprises a rate integrator that is configured to: sample apump control signal, create a sampled pump control signal, and sum thesampled pump control signal to determine when the first amount ofdeionized water has been delivered to the pressurized container and todetermine when the second amount of concentrated sheath fluid has beendelivered to the pressurized container.
 3. The system of claim 1wherein: the pressurized reservoir is configured to flow the mixture outof the pressurized reservoir at an outflow rate, and the pump isconfigured to cause deionized water in the first container andconcentrated sheath fluid in the second container to flow to thepressurized reservoir at a flowrate that is substantially equal to theoutflow rate.
 4. The system of claim 3 further comprising: a rinse pumpfluidically connected to the first container; a second valve fluidicallyconnected to the rinse pump; and a nozzle of a flow cytometer that isfluidically connected to the second valve, wherein: the rinse pump isfluidically interposed between the second valve and the first container,the second valve is fluidically interposed between the nozzle and therinse pump, the rinse pump is configured to cause deionized water in thefirst container to flow through the second valve to the nozzle.
 5. Thesystem of claim 1, wherein the concentration of sheath fluid in thepressurized reservoir is nominally seven parts deionized water to onepart concentrated sheath fluid.
 6. The system of claim 1, wherein thefirst container contains deionized water and the second containercontains concentrated sheath fluid.
 7. The system of claim 1, whereinthe flowrate of the pump is about 8 milliliters per minute.
 8. Thesystem of claim 1, wherein the first amount is about 7 milliliters ofdeionized water and the second amount is about 1 milliliter ofconcentrated sheath fluid.
 9. The system of claim 2, wherein thecontroller is further configured to, based on the determination,generate a valve control signal to cause the valve to switch between thefirst position and the second position.
 10. The system of claim 3,further comprising a level sensor communicatively connected to thecontroller and configured to generate a level sensor signal that isrepresentative of a level of the mixture in the pressurized reservoir,wherein the controller is further configured to, based on the levelsensor signal from the level sensor, change the flowrate of the pump.11. The system of claim 10, wherein the controller is further configuredto cause, based on a determination that the level of the mixture in thepressurized reservoir is below a first level, the flowrate of the pumpto increase.
 12. The system of claim 10, wherein the controller isfurther configured to cause, based on a determination that the level ofthe mixture in the pressurized reservoir is above a second level, theflowrate of the pump to decrease.
 13. The system of claim 4, furthercomprising one or more of: an injector needle, a sample tube, and asample uptake tube, wherein: each of the injector needle, the sampletube, and the sample update tube are fluidically connected to the secondvalve, and the rinse pump is configured to cause deionized water in thefirst container to flow through the second valve to the one or more of:the injector needle, the sample tube, and the sample uptake tube. 14.The system of claim 4, further comprising a waste collector fluidicallyconnected to the nozzle, wherein: the nozzle is fluidically interposedbetween the second valve and the waste collector, and the rinse pump isfurther configured to cause deionized water in the first container toflow through the second valve, through the nozzle, and to the wastecollector.